Pultrusion is a cost effective manufacturing process for producing continuous runs of constant cross section structural members made from fiber reinforced composite material, particularly those made using thermoset and thermoplastic matrix materials. The details of a particular pultrusion process implementation vary depending on the specific materials being converted to useful structures and the shape of the structures being produced. In general, in a typical pultrusion process, a succession of processing operations is arranged one after the other in series and designed to function together as a single, continuously flowing stream, with each step of the process automatically feeding the next with a steady flow of material. In one implementation, dry materials, in the form of individual tows of fibers (i.e., like thread on a spool) and/or fabrics of the same or different fiber on creels are continuously fed into a set of guides that form the materials into the general shape of the finished components. The materials are then fed into a station that completely wets the dry fiber materials with the matrix resin. The wet materials then enter the pultrusion die, in which the resin reacts or cures to a solid material. Curing may continue with additional heaters downstream of the die exit. A pulling mechanism is used to move the material continuously through the process at a steady pace. The production line may end with a cutting mechanism to cut the finished product to predetermined lengths.
The pultrusion process requires highly polished die surfaces that are often chrome (or other metal) plated in order to provide the combination of low surface friction and wear resistance required to successfully pass composite product through the die without excessive friction or product adhesion to the die surface.
A pultrusion die is usually made with multiple parts, each part typically being made from a precisely machined and polished, structural, heat transmitting, high quality, costly tooling steel. Usually top and bottom die components are the largest parts of the pultrusion tool, and span a pultrusion machine""s width. The interior surfaces of the die components facing a die cavity generally have a high degree of surface polish, and often a mirror surface. The cost of both the high quality tooling materials and the polishing needed to achieve surfaces with the required surface finish makes conventional pultrusion tooling relatively expensive. Tool cost increases as tool size increases, and can often be prohibitive, particularly for short prototyping runs of large parts.
One application of the pultrusion process is the production of sandwich panels made with foam core and thin composite skins. In one example of how a sandwich panel might be pultruded, sheets of core, often in the form of a homogeneous closed-cell foam, that have been cut to the proper thickness and width are butted edge-to-edge so that no significant gap exists between the trailing edge of the first-to-be-introduced foam sheet and the leading edge of the next-introduced sheet of foam. These sheets are introduced between upper and lower skins of fiber fabric at any point before the entrance to the pultrusion die. The foam then moves through the process with the skins. The closed cell foam prevents resin impregnation into the cores. The finished part exits the die as two rigid cured composite face sheets laminated to the thicker, lightweight core.
A limitation to the size of finished parts produced by the pultrusion process is the cost to fabricate large dies. As the components being considered for pultrusion become ever larger, the costs associated with providing such high quality surfaces on large steel plates becomes increasingly larger, due to the quality of steel required and to handling difficulties with the large plates (possibly greater than 10 feet wide and more than 3 inches thick, weighing many thousands of pounds).
A low cost alternative to conventional pultrusion tooling methods achieves a highly polished pultrusion die surface by replacing the conventional machining and polishing of expensive tooling steel with lining some or all of the die""s interior surfaces with highly polished, low cost, commercially available sheet steel. Using this technique, the component surfaces, that do not form the cavity, and possibly the mating surfaces of the die components, are optionally finished to less exacting, less expensive levels. Through holes, tapped holes and tooling pins are drilled in the components so that the die components can be fastened together with the polished surfaces facing the interior and forming the cavity. When the entrance and exit orifices have rounded or beveled edges, the sheeting material conforms to the adjacent plate surfaces, allowing material to feed into the die more smoothly and the finished pultruded product to release from the tool more easily. Heaters and other components needed for the pultrusion process are added to the die before or after it is mounted on the pultrusion machine. Side edges with features such as flanges or recessed regions are formable with the polished sheeting die system. The sides of the cavity can be made using more conventional tooling techniques and integrated with other tooling components lined with polished sheet to form a complete pultrusion die. Alternatively, the side pieces of the cavity can be formed less expensively and lined with polished sheet before being integrated with the other components. The technique works well for large, flat panel dies, and can also be used to make tools with rounded surfaces, multiple cavities, multiple curvatures and other complex cross sections. Other aspects, features, and advantages of the present invention are disclosed in the detailed description that follows.